arm64: defconfig: updates for 4.1

Enable a few useful options in our defconfig:

  - New platform support (exynos7, seattle, tegra132)
  - SKY2 (ethernet in newer revisions of Juno)
  - Xgene reboot support
  - Virtio-pci for kvmtool and qemu
  - EFIVAR_FS (previously selected as a module)
  - NFSv4

Signed-off-by: Will Deacon <will.deacon@arm.com>